1. Field of the Invention
The present invention relates to an image reading method and device for use in an image scanner or a facsimile machine.
2. Description of Related Art
It is known that general image reading devices which are used in image scanners, facsimile machines or the like employ reduced optical systems of the type shown in FIG. 33. Such an image reading device includes a longitudinally elongated light source 3102 which illuminates an original 3101, a mirror 3103 which bends an optical path for reducing the size of the entire device, a lens 3104 which focuses light information from the original 3101, a white reference correction plate (shading correction plate) 3105 which corrects a degradation due to a cosine fourth law of the white output waveform of the lens 3104, and a CCD (image pickup element) 3106 which converts the light information focused by the lens 3104 into an electrical signal. An image signal which is outputted from the CCD 3106 is A/D converted by an A/D (analog/digital) converter, and is subjected to image processing.
In some types of image reading devices having an arrangement similar to the above-described one, an LED (light-emitting diode) is used as a light source for illuminating an original.
However, since the absolute amount of light per LED is small, there is the problem that if stable image information is to be obtained, it is necessary to extend the storage time required for image reading or to increase the sensitivity of a reception system.
However, at present, although research and development for increasing the driving speed of a BASIS sensor (base-stored image sensor) or a CCD device which serves as a light-receiving element has been promoted, the sensitivity characteristics of a light-receiving part for photoelectrically converting light information of an original are limited, and if an S/N (signal/noise) ratio is to be improved, it is necessary to illuminate the original with a sufficient amount of light.
If driving current which flows through the LED and the like is increased to increase the amount of light, there is the problem that a variation of the current due to a variation in the wavelength of light and a variation in forward voltage occurs as the result of the generation of heat in the LED itself and a stable amount of light cannot be obtained.
Two methods for solving this problem have heretofore been known. One method called preheating is to quickly heat a light source up to a predetermined temperature in advance, while the other method called warmth retention is to hold a predetermined constant state even while scanning is not being performed. These methods are capable of eliminating a thermally unstable state of the light source immediately after a power supply is turned on, and a thermally unstable factor immediately after a scan is started after a long interrupt.
However, if colors are to be read by sequentially switching LEDs for three different colors R, G and B, particularly if the amount of current which flows through each of the color LEDs greatly varies during each color scanning, heat equilibrium which contains the self heating of the LEDs themselves is impaired, and heat is gradually accumulated in the LEDs and an unbalance occurs between the start and the end of scanning of an original. As a result, an influence such as a subtle variation in color or contrast is exerted on an image.
An object of the present invention is to provide an image reading device capable of obtaining stable high-quality image information as well as a method of controlling the same.
To achieve the above object, in accordance with one aspect of the present invention, there is provided an image reading device which reads an original image illuminated by a light source, by using a light-receiving element, the image reading device comprising control means for controlling energy consumption of the light source so that energy which is consumed by the light source in the process of obtaining reference data which serves as a reference for correcting an image signal outputted from the light-receiving element is made approximately the same as energy which is consumed by light source in the process of obtaining no reference data.
In accordance with another embodiment of the present invention, there is provided a method of controlling an image reading device which reads an original image illuminated by a light source, by using a light-receiving element, the method comprising a step of controlling energy consumption of the light source so that energy which is consumed by the light source in the process of obtaining reference data which serves as a reference for correcting an image signal outputted from the light-receiving element is made approximately the same as energy which is consumed by the light source in the process of obtaining no reference data.
According to the above-described arrangement and construction, it is possible to obtain accurate reference data and hence stable high-quality image information.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.